(1) Field of the Invention
This invention relates to analog mixers or multipliers, in particular, to electronic mixers or multipliers using bipolar transistors. Such mixers are used for multiplying two input signals or for converting two difference frequency signals into a beat frequency output.
(2) Brief Description of Related Art
In a conventional radio receiver, the incoming radio frequency is mixed with a local oscillator (LO) signal to produce a beat frequency, which is the intermediate frequency (IF). The IF is then amplified and filtered to attenuate other unwanted signals,
A popular mixer circuit is the differential pair or Gilbert multiplier. A bipolar transistor version of the Gilbert multiplier is shown in FIG. 1. Basically, a differential amplifier with a differential pair Q2 and Q3 is fed from a current source Q1. The differential gain of the differential amplifier is proportional to the transconductance gm of Q2 and Q3. This transconductance varies as the dc collector current of Q2 and Q3, which is controlled by the collector current of Q1. The dc collector current IC1 of Q1 is controlled by the dc base current VB1 of Q1. When a local oscillator signal VLO of frequency fLO is applied differentially to Q2 an Q3 (i.e. VLO+=Vinxe2x88x92/2 and VLOxe2x88x92=Vin+/2 respectively), and a radio frequency signal Vrf of frequency frf is applied to the base of Q1, the output current of the differential amplifier is equal to VLO*gm2, and the gm2 is proportional to Vrf*gm1. When the Vrf is multiplied by VLO, a beat frequency fif=frfxc2x1fLO intermediate frequency signal Vif is produced.
While the Gilbert multiplier is widely used, it has a number of drawbacks for low voltage and low power applications. In modern integrated circuit technology, the tendency is to use a low supply voltage VCC: for instance 25 V for 0.25 xcexcm technology and 1.8 V for 0.18 xcexcm technology. In the Gilbert mixer, the current source is operating in the active region of the VCE1 vs IC1 V-I characteristic Q1 in FIG. 2 to obtain a higher transconductance and is proportional to Vrf. Therefore the dc collector voltage VC1 for the current source Q1 must not forward bias the collector junction, i.e. about the same as the dc base voltage (≈0.7V for silicon BJT) to operate in the active region.
Similarly, the differential pair Q2 and Q3 also must have its collector voltage high enough to operate in the active region. Since the dc base voltage is about 0.7V higher than the emitter voltage of Q2 or VC1 (≈0.7V), the dc base voltage of Q2, hence the dc collector voltage, must be at least about 2xc3x970.7V or 1.4V. This voltage is higher than certain kind of battery voltage of 1.3 V.
If a resistor is used as a load, another voltage drop VL will be added to be supplied by the power supply VCC. These three stacks of voltages, VCE1, VCE2 and VL, dictate that the supply voltage cannot be made very low. For a typical base-to-emitter voltage of 0.7 V, there is hardly any xe2x80x9chead roomxe2x80x9d for signal voltage swing. Besides, a higher supply voltage consume more power.
In U.S. Pat. No. 6,388,501, Chien disclosed an MOSFET mixer operating at the knee of the drain V-I characteristics of the MOSFET. Since a MOSFET is gate voltage control device while a BJT is a base current control device and since the drain and source characteristics of a MOSFET are symmetrical but the collector and emitter characteristics of a BJT are not symmetrical, Chien""s circuit is not applicable to a BJT mixer.
An object of this invention is to design a BJT mixer which requires a lower supply voltage than the Gilbert mixer or similar structure. Another object of this invention is to reduce the power consumption of the BJT mixer. Still another object of this invention is to provide a high conversion gain of the mixer. A further object of this invention is to set the operating point of the mixer at its optimum conversion gain automatically
These objects are achieved by mixing the RF signal and the local oscillator signal at the knee of the output VCE-IC characteristic of a BJT as shown in FIG. 2 with constant base current IB1. At the knee, the characteristic has the sharpest curvature. The nonlinearity produces a maximum beat frequency signal. For implementation, a mixer BJT is biased at the knee of the VCE-IC characteristic. The LO (or RF) signal voltage VLO (or Vrf) is applied at the base of the mixer BJT, and the RF (or LO) signal voltage Vrf(or VLO) is injected at the collector of the mixer BJT. Then a beat frequency drain current is produced. Specifically, the gate of a single-ended mixer BJT is fed with a local oscillator signal and the collector of the mixer is dc coupled to a single-ended emitter follower with the base fed from a radio frequency signal or vise versa.